In order to provide a rubber product such as a transmission belt and a tire with tensile strength and dimensional stability, it is common practice to embed a high-strength fiber such as a glass fiber, a nylon fiber and a polyester fiber as a reinforcement in a parent rubber. A rubber-reinforcing fiber, for use as the reinforcement embedded in the parent rubber, needs to have good adhesion to the parent rubber to define a tight interface on which separation does not occur. The glass fiber itself cannot however be adhered to the parent rubber and, even if adhered, shows such weak adhesion as to cause interfacial separation and fails to function properly as the reinforcement.
Therefore, as the rubber-reinforcing glass fiber used by being embedded in the parent rubber in production of the transmission belt, those who produced by coating a glass fiber cord with a coating liquid for improving adhesion to the parent rubber are employed. More specifically, the rubber-reinforcing glass fiber commonly used for improving adhesion between the parent rubber and the glass fiber cord and for preventing the interfacial separation is produced by applying a coating liquid in which a resorcinol-formaldehyde resin and various latex components are dispersed in water to a glass fiber cord of filament yarn and then drying the glass fiber cord to form a coating layer.
The coating layer has the effect of making adhesion between the parent rubber and the glass fiber cord, when the rubber-reinforcing glass fiber is embedded in the parent rubber at high temperatures to form the transmission belt; however, adhesiveness, i.e. adhesive strength is not enough. For example, a heat-resistant rubber such as hydrogenated nitrile rubber (cross-linked with sulfur or peroxide and hereinafter abbreviated as “HNBR”) is employed as the parent rubber of an automotive transmission belt for use in a high-temperature engine room environment. In the case where the rubber-reinforcing glass fiber is treated only with the above coating process and embedded, however, the transmission belt cannot maintain an initial adhesion strength during running where the transmission belt is continuously bent under high-temperature conditions. This can result in the occurrence of interfacial separation between the rubber-reinforcing glass fiber and the parent rubber during long hours of running.
Patent Documents 1 to 4 disclose rubber-reinforcing glass fibers for use in transmission belts which can maintain adhesion between the rubber-reinforcing glass fibers and HNBR without causing interfacial separation therebetween to secure long-term reliability even under high-temperature running conditions, by performing the above coating process to form a first coating layer on a glass fiber cord and then by applying a second coating liquid of different compositions to a second coating layer and drying to form the second coating layer.
For example, Patent Document 1 discloses a technique for making a coating treatment on the glass fiber cord to produce the rubber-reinforcing glass fiber, the technique using a second coating liquid containing a halogen-containing polymer and an isocyanate.
Patent Document 2 discloses a rubber-reinforcing glass fiber on which first and second coating layers are formed, the first coating layer being formed by applying a treating agent containing a resorcin.formalin condensate and rubber latex to a rubber-reinforcing glass fiber while the second coating layer is formed by applying a treating agent of different compositions to the first coating layer. This rubber-reinforcing glass fiber is characterized in that the treating agent for the second coating layer contains a rubber blended material, a curing agent and a curing auxiliary as main components.
Patent Document 3 filed by the present applicant discloses a rubber-reinforcing glass fiber having a coating layer and a further coating layer. The coating layer is produced by drying a coating liquid for coating glass fibers after applying it to a glass fiber cord, the coating liquid being an emulsion in which an acrylic ester resin, a vinylpyridine-stylene-butadiene copolymer and a resorcinol-formaldehyde resin are dispersed in water. The further coating layer is formed by applying a further coating liquid for coating glass fibers to the coating layer, the further coating liquid being produced by dispersing a halogen-containing polymer and a bisallylnagiimide in an amount of 0.3 wt % to 10.0 wt % relative to the halogen-containing polymer in an organic solvent. This rubber-reinforcing glass fiber exhibits good adhesive strength in adhesion to HNBR.
Patent Document 4 filed by the present applicant discloses a rubber-reinforcing glass fiber produced having first and second coating layers. The first coating layer is formed by applying a first coating liquid for coating glass fibers to a glass fiber cord to form a coating film and then drying and curing the coating film. The second coating layer is formed by applying a second coating liquid of different compositions to the first coating layer to form a coating film and then drying and curing the coating film. This rubber-reinforcing glass fiber is characterized in that the second coating liquid is obtained by dispersing bisallylnagiimide, a rubber elastomer, a vulcanization agent and an inorganic filler in an organic solvent. This rubber-reinforcing glass fiber shows good adhesive strength in adhesion to HNBR and, when embedded in HNBR for use in the transmission belt, shows high heat resistance without reduction in tensile strength even after long hours of running under high-temperature conditions.
Conventionally, an automotive timing belt used as a heat resistant transmission belt reinforced with a rubber-reinforcing glass fiber had been produced to have a coating layer and a second coating layer formed on the coating layer, and had been embedded in a heat resistant rubber such as HNBR, the coating layer being formed by applying a coating liquid containing a resorcin.formalin condensate as an essential component to a glass fiber cord and drying it, the second coating layer being formed by applying a coating liquid of different components to the coating layer and drying it.
The above conventional transmission belt secures initial strength of adhesion between the rubber-reinforcing glass fiber produced by applying the coating material to the glass fiber cord and the parent rubber, but does not secure both high water resistance and high heat resistance and does not maintain a tensile strength without changes in dimension even after long hours of running under high-temperature and high-humidity conditions.
Therefore, much further developments of the transmission belt and of the rubber-reinforcing glass fiber are desired. More specifically, the transmission belt is required to have an equal or superior adhesive strength between the rubber-reinforcing glass fiber and a heat resistant rubber, an equal or superior water resistance with which the coating layer can maintain the initial adhesive strength even with dropping of water on the transmission belt, and an equal or superior heat resistance with which the coating layer can maintain the initial adhesive strength even after long hours of running under high-temperature high-humidity conditions, as compared to the conventional transmission belt produced by embedding the rubber-reinforcing glass fibers as discussed in Patent Documents 1 to 4 in the heat resistant rubber.
Patent Document 5 filed by the present applicant discloses a coating liquid for coating glass fiber cords, in which an emulsion is formed by dispersing a monohydroxybenzene-formaldehyde condensate, a vinylpyridine-styrene-butadiene copolymer and a chlorosulfonated polyethylene in water.
Patent Document 6 filed by the present applicant discloses: a rubber-reinforcing glass fiber provided with a first coating layer formed by applying the coating liquid as disclosed in Patent Document 5, and a second coating layer formed on the first coating layer and containing a halogen-containing polymer and bisallylnadiimide; a rubber-reinforcing glass fiber provided with a second coating layer formed on the first coating layer which second coating layer contains a halogen-containing polymer and a maleimide; a rubber-reinforcing glass fiber provided with a second coating layer formed on the first coating layer which second coating layer contains a halogen-containing polymer, an organic diisocyanate and zinc methacrylate; and a rubber-reinforcing glass fiber provided with a second coating layer formed on the first coating layer which second coating layer contains a halogen-containing polymer and a triazine compound.
Further, Patent Document 7 discloses an impregnant for glass fibers, containing a resorcin-chlorophenol-formaldehyde resin. The resorcin-chlorophenol-formaldehyde resin is a water-soluble addition condensate obtained by reacting resorcin, chlorophenol and formaldehyde in the form of an aqueous solution, and is available from ICI under the trade name of “Vulcabond E” as an aqueous solution containing water-soluble resorcin-chlorophenol-formaldehyde resin and having solid content of about 20 wt %.
Patent Document 1: Japanese Examined Patent Publication No. 2-4715
Patent Document 2: Japanese Laid-Open Patent Publication No. 11-241275
Patent Document 3: Japanese Laid-Open Patent Publication No. 2004-203730
Patent Document 4: Japanese Laid-Open Patent Publication No. 2004-244785
Patent Document 5: Japanese Laid-Open Patent Publication No. 2006-104595
Patent Document 6: International Publication 2006/038490 Pamphlet
Patent Document 7: Japanese Laid-Open Patent Publication No. 3-65536